Beyond the iron age: the ecological relevance of non-ferrous bioactive trace metals and organic growth factors in aquatic systems

Citation: Gómez-Consarnau L and Sañudo-Wilhelmy SA (2015) Beyond the iron age: the ecological relevance of non-ferrous bioactive trace metals and organic growth factors in aquatic systems. Front. Microbiol. 6:218. The catalysis of chemical reactions is one of the most central processes in biology, as most reactions in living organisms would occur too slowly to play any role in their metabolism. For example , the rate of biologically relevant reactions would take hundreds to millions of years in the absence of a catalyst (Stockbridge et al., 2010). Enzymes are the catalysts of nature, and they can increase the rate of a chemical reaction by as much as 10 20 times compared to an uncat-alyzed reaction in water (Lad et al., 2003). Molecular evidence suggests that enzymatic cataly-sis evolved very early in life history, even prior to the Archaean genetic expansion (David and Alm, 2010). While some protein functional groups can act as catalysts, they represent a very limited set of reactions as compared to the multitude of enzyme-catalyzed reactions (Price and Stevens, 1999; Bugg, 2012). Enzymes utilize a variety of non-protein molecules or metal ions to increase their versatility in catalytic capacity. Coenzymes and cofactors are the non-protein components of the enzymes that help catalyze the reactions. Coenzymes are typically vitamin B derivatives while cofactors are molecules or metal ions that are covalently bound to the enzyme (Broderick, 2001). The multiple oxidation states of some metals increase the functionality of the enzymes. Most of the first row transition metals as well as Mo, W, and Mg are known to function as cofactors in enzymatic catalysis (da Silva and Williams, 2001). Fe is the most abundant transition element in the Earth's crust and, although it is very scarce in the ocean, this element was very abundant at the onset of life (Anbar, 2008). Therefore, it is not surprising that Fe is a major cofactor in some of the most central processes in different biological systems (da Silva and Williams, 2001). Starting with the pioneering work of John Martin and collaborators in the early 1990s (Iron limita-tion?, 2000; Martin et al., 1991), the so-called iron hypothesis has produced major breakthroughs in our understanding of the fundamental role it plays in the marine carbon cycle (Smetacek et al., 2012; Thiele et al., 2012). However, biological activity is not entirely sustained by Fe alone as other elements are used as cofactors in relevant metabolic reactions …